The present invention is directed to an improved graphite mold for casting. More specifically the invention provides a method and apparatus to improve riser practice and control of the casting process especially in large castings to reduce porosity in these castings and thus improve their internal structural integrity.
Castings have been produced for centuries in various molds and forms, including sand molds, permanent dies and graphite molds. The materials cast include molten bronze, brass, iron, aluminum and steel, as well as other elements and alloys. Casting techniques have developed to accommodate the properties of the material being cast, the required or desired surface quality of the finished casting, the requisite structural integrity of the cast product, as well as the cost of the casting technique or practice. Indicative of some of the first casting practices was the tapping of molten iron from a crude smelter or blast furnace into a mold formed in the sand or dirt in Biblical times. Casting practices have evolved and been refined to provide better control of the melt chemistry, tap temperature, mold temperature, pouring rates, ladle apparatus as well as other casting and teeming techniques. Among the more progressive practices is the selection of mold materials to promote longer wearing dies, to yield improved control of heat transfer rates, and to provide smoother surfaces on the as-cast product.
Graphite molds are utilized in the casting and foundry industry for their efficient heat transfer properties, and their relatively high resistance to wear and erosion, which leads to an increase in the number of casts per mold and more rapid cooling of the cast articles in the mold. These graphite molds are also utilized in the foundry industry to provide greater dimensional control of the as-cast product, which reduces secondary finishing operations and provides a product ready for shipment. As in most foundry mold and casting apparatus, these graphite molds are provided with pouring gates and risers, which permit discharge of gases and vapors during casting and solidification. The high heat transfer rate of the graphite is both a boon and a bane, as it allows for rapid heat transfer from the molten metal, but can also lead to an increase in the occurence of porosity in the solidified casting. Therefore, it has been found to be desirable to not only control the mass rate of cooling, but also to locally control the rate of cooling in the various regions of the casting. That is, it is beneficial to vary the cooling rate for both the thinner and the thicker casting sections to promote a more uniform rate of cooling and solidification through each of the sections of the casting in the mold.
Molds, mold structures and mold practices vary across a wide range of apparatus, as exemplified by structures discussed and illustrated in Foundry Work, by R.E. Wendt, Fourth Edition, 1942, and Cast Metals Technology, by J.Gerin Sylvia, 1972. Although these texts span three decades, many of the methods and apparatus are the same or similar. The improvements in the technology noted in the latter text relate to an understanding of the kinetics of the operation and the chemical practices, however, the mold components have largely remained the same, that is the gate, riser, mold cavity, cope and drag among others. Although these basic components remain part of the art, there have been continuous efforts to improve the casting practice, mold structure and metal chemistry with the intent of improving the finished product, its internal structure and the finished surface.
U.S. Pat. No. 3,614,053 to Peck is illustrative of efforts to improve casting and discloses a riser assembly for a two-part mold, which riser has a pair of sections resiliently and pivotally mounted on the respective mold parts. As the mold parts are joined to provide a casting cavity the riser pair is simultaneously coupled to provide a sealed riser cavity. This arrangement was utilized to expedite and enhance the casting cycle rate through the mold.
Alternative mold riser structures are taught and illustrated in U.S. Pat. Nos. 3,409,267 to Wszolek; 3,498,366 to Merrick et al; and, Re. 24,655 to Sylvester. The latter patent to Sylvester provides a technique for forming and baking riser cups in the riser openings of molds. These cups are a nonflammable material such as core sand and dry binder baked to provide a smooth strong cup with walls, which will vent gas from the reaction between the molten metal and the graphite. The Merrick et al--'366 patent has resin-bonded sand portions in a graphite mold to provide improved surface conditions of the as-cast articles, and to produce chilled wear surfaces at the graphite contacting portions as well as softer surfaces at the sand contacting portions of the cast article. A riser with a separate, relatively large and reusable upper section is illustrated in the noted Wszolek patent.
Other casting and foundry techniques utilized to prolong mold life and to produce improved castings are exemplified by the utilization of mold washes to coat the casting surfaces of the mold, such as taught in U.S. Pat. No. 3,684,004 to Germain et al. On a mold face with a centrally located ingate, a mold-wash coating thickness varies inversely with the distance from the centerline of the mold. These mold washes are generally applied by spraying the mold casting surfaces with a slurry of coating material, such as quartz, zircon, cristobalite or the like. This patent provides a means to avoid the laps, wrinkles and discontinuities from the mold-coating process.
The desire for improved cast products extends to all facets of the casting, that is the chemical composition, crystallographic structure, physical properties, surface finish, minimal material losses, grinding requirements, and structural integrity. This latter parameter is especially important in castings subject to mechanical work and wear, or which castings may bear either intermittent or continuous heavy loads. This improved quality requirement is exemplified by cast railroad wheels, which are subject to wear, heavy vertical and torsional loading, and abrasion. These difficult physical requirements place a premium on the provision of a quality casting, and one of the primary characteristics of a structurally sound casting is the continuity or the minimization of microporosity in the casting. This casting soundness is monitored by ultrasonic testing and is indicative of, or at least considered to be an indicator of, a high quality railroad wheel for the strenuous service requirement. Earlier casting practice measures to improve the structural integrity of the cast railroad wheels have included an increase in the number of risers, as well as positioning the risers in communication with the web section of the mold cavity to improve the gas and vapor discharge and to provide a source of hot metal to fill the voids as the casting cools and contracts. It has been considered necessary to continuously fill the casting cavity with hot metal to minimize microporosity in the castings.